1. Field of the Invention
This invention relates to a vacuum glass envelope formed of at least two glass substrates and more particularly to an envelope suitable for use for a display device.
2. Discussion of the Background
An envelope for a display device in which an electron emission source and illuminants or luminous elements such as phosphors or the like is generally constructed in the form of a vacuum glass envelope. Conventionally, the vacuum glass envelope is formed of a first glass substrate and a second glass substrate arranged in a manner to be opposite to the first glass substrate and provided therein with illuminants or luminous elements providing a display plane and an electron source for exciting the illuminants for luminescence. The first and second glass substrates are arranged so as to be spaced from each other at a predetermined interval by means of struts interposedly arranged therebetween.
Such construction of the conventional envelope is as shown in FIG. 7, wherein struts 101 are interposedly arranged between a first substrate 102 and a second substrate 103. The struts 101 each are formed into a diameter of about 50 .mu.m and a length of about 200 .mu.m.
Now, an arrangement of the struts 101 between the first substrate 102 and the second substrate 103 will be described hereinafter with reference to FIGS. 6(a) to 6(c).
First, a glass fiber which is, for example, about 50 .mu.m in diameter which is a material for each of the struts 101 is cut into a length of, for example, about 200 .mu.m.
The struts 101 thus cut each are then washed and subject to positioning on the first substrate 102 by means of a fixture 110.
The fixture 110, as shown in FIG. 6(a), is formed into a box-like shape and includes an alignment plate 111 provided with strut holes 114 for receiving the cut struts 101 therein while raising them, a porous section 112 made of a porous material and arranged under the alignment plate 111, and an evacuation section 113 through which the fixture 110 is evacuated.
The strut holes 114 are formed on the alignment plate 111 in a manner to positionally correspond to arrangement of the struts 101 on the first substrate 102.
In the fixture 110 thus constructed, the evacuation section 113 is connected to a pump (not shown) to cause the interior of the fixture 110 to be evacuated therethrough, during which the cut struts 101 are spread on the alignment plate 111 of the fixture 110. This causes gas sucked though the strut holes 114 to be passed through the porous section 112 and evacuated from the evacuation section 113, so that the cut struts 110 each are caused to enter, by suction, the strut hole 114 formed into a diameter somewhat larger than that of the strut 101 while being raised, resulting in being arranged in the strut hole 114 while being kept raised, as shown in FIG. 6(a).
Then, as shown in FIG. 6(b), a glass substrate 120 to which a transfer paste 121 is applied is placed on the fixture 110 while keeping the paste 121 facing the fixture and then contacted with the struts 101 held in the fixture 110, so that the transfer paste 121 may be transferred to an upper or one end surface of each of the struts 101.
Subsequently, as shown in FIG. 6(c), the first substrate 102 is placed on the fixture 110 in which the struts 101 having the transfer paste 121 thus transferred thereto are held while being aligned with the fixture 110, so that the struts 101 are attached at the one end surface thereof to the first substrate 102. Then, the first substrate 102 is subject to calcination at a predetermined temperature to melt the transfer paste 121, to thereby fix the one end surface of each of the struts 101 to the first substrate 102 by welding.
The transfer paste 121 mainly consists of a sealing glass of a low softening point which has lead oxide incorporated therein so as to permit a thermal expansion coefficient of the transfer paste 121 approach to that of the first substrate 102 made of glass and may optionally contain a solvent and the like, resulting in exhibiting stickiness.
Thereafter, the second substrate 103 is arranged in a manner to be opposite to the first substrate 102 to which the struts 101 are fixed in a manner to be spaced from each other at predetermined intervals, resulting in a glass envelope being provided. Then, the glass envelope is evacuated to vacuum, so that a vacuum glass envelope in which the struts 101 are firmly held between the first substrate 102 and the second substrate 103 by an atmospheric pressure is provided as shown in FIG. 7.
Alternatively, the vacuum glass envelope may be prepared in a different way. More particularly, the transfer paste 121 is transferred to the other end surface of the struts 101 and the first and second substrates 102 and 103 are laid on each other while being aligned with each other. Then, the substrates 102 and 103 are placed in an oven, resulting in being sealedly joined to each other. Concurrently, this causes the transfer paste 121 to be melted, to thereby fix the other end surface of the struts 101 to the second substrate 103, so that the glass envelope may be provided in which the struts 101 are arranged between the substrates 102 and 103.
In order to ensure that the first substrate 102 and second substrate 103 are arranged in a manner so as to be opposite to each other at a predetermined interval, side plates of a predetermined height, glass beads, glass fibers or the like are interposed therebetween while being positioned at a periphery thereof. Then, the above-described sealing operation is carried out for providing the glass envelope.
The struts 101 are arranged in the glass envelope in a manner to be spaced from each other at intervals of about 2 to 5 mm.
Arrangement of the struts 101 in the strut holes of the alignment plate 111 which is carried out as described above is shown in FIGS. 8(a) and 8(b). As will be noted from FIG. 8(a) and particularly FIG. 8(b), it is often seen that the struts 101 are each arranged in the strut hole while being inclined or positionally deviated with respect to the strut hole.
One of the reasons is that when the strut hole into which the strut 101 is inserted is formed into a diameter of about 53 .mu.m in order to permit a slight gap which is sufficient to prevent the strut 101 from being caught in the hole to be provided between the strut hole and the strut, the strut 101 and strut hole are caused to have tolerances as large as +3 .mu.m and +5 to 0 .mu.m, respectively.
Another reason is that preparation of the alignment plate 11 with increased accuracy fails to form it into a thickness of about 70 .mu.m or more, to thereby cause the struts 101 to be projected from the alignment plate 111 by a distance as large as about 130 .mu.m.
Thus, the struts 101 are each caused to be positionally deviated or misregistered with respect to the strut hole by a magnitude up to about 20 .mu.m. When the vacuum glass envelope is used for a display device in which a picture cell pitch and a picture cell interval are respectively defined to be 360 .mu.m and 80 .mu.m, such misregistration in an amount as large as about 20 .mu.m causes the strut 101 to interrupt display operation of a display section or leads to short circuit of electric wirings in the display device.
The reason why the conventional vacuum glass envelope fails to form the alignment plate 111 into a thickness of about 70 .mu.m or more is due to the way in which the alignment plate is prepared.
Now, preparation of the alignment plate 111 will be described hereinafter with reference to FIGS. 9(a) to 9(d), as well as FIGS. 10(a) and 10(b).
First, a photoresist 132 is applied to a stainless steel plate 131 as shown in FIG. 9(a) and then a glass dry plate 133 having a mask 134 wholly formed thereon is placed on the photoresist 132, followed by irradiation of light to the glass dry plate 133, to thereby subject the photoresist 132 to exposure, as shown in FIG. 9(b). Then, the photoresist 132 is developed, resulting in only portions 135 of the photoresist exposed to light being left on the stainless steel plate 131, as shown in FIG. 9(c).
Such photoresist portions 135 thus left are formed in large numbers on the stainless steel plate 131 and positions of the photoresist portions 135 left on the plate 131 are determined to be positions at which the struts 101 are to be arranged while being raised.
Then, the stainless steel plate 131 is subject to electroplating while using Ni as a plating material, so that a Ni plate 136 is formed on only the stainless plate 131 as shown in FIG. 9(d).
Thereafter, the photoresist 135 is removed from the stainless steel plate 131, resulting in the Ni plate 136 which is formed with a strut hole 137 being formed on the stainless steel 131, as shown in FIG. 10(b). Then, the Ni plate 136 is peeled off from the stainless steel plate 131, so that the alignment 111 is provided which is formed with a plurality of such strut holes 137 as shown in FIG. 10(b).
The thickness of the alignment plate 111 may be increased by increasing a thickness of the photoresist 132 applied to the stainless plate 131. Unfortunately, such an increase in thickness of the photoresist 132 substantially prevents light irradiated for exposure from reaching the depths of the photoresist 132, leading to a failure to fully expose it to light. Also, the light irradiated is diffracted at a boundary between the mask 134 and the photoresist 132 and scattered in the photoresist 132, therefore, an increase in thickness of the alignment plate 111 causes light to be gradually diffused as it penetrates through the photoresist 132. This causes the photoresist 135 subjected to exposure as shown in FIG. 9(b) to have a configuration like a trapezoid-like shape, so that a variation in dimension of the strut hole 137 formed is increased with an increase in thickness of the alignment plate 111, as shown in FIG. 11.
Such an increase in variation of dimension of the strut hole or formation of the strut hole into a trapezoid-like shape undesired further promotes misregistration of the strut 101 from a center of the strut hole to a degree sufficient to cause the vacuum glass envelope to be unsuitable for use for a display device.
Thus, it will be noted that the prior art fails to increase a thickness of the alignment plate 111 to about 70 .mu.m or more while ensuring formation of the strut holes with satisfactory accuracy.